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What are worm casts?

Worm casts occur on the surface of lawns and are small heaps of muddy soil ejected from the digestive tract of some species of earthworm, mainly Allolobophora species, such as the green worm A. chlorotica.

Symptoms

Earthworms are soil-dwelling animals that have elongate cylindrical segmented bodies. Adult earthworms have a distinctive thickened band, known as the clitellum, about one third of the way down the body from the head end, this is part of the reproductive system of the worm.

In general earthworm activities are beneficial in gardens, for both soil structure and in nutrient recycling. Earthworms feed on dead plant material and in doing so ingest a certain amount of soil. As a consequence, an earthworm's excrement has a muddy consistency and appearance. Most earthworm species void waste material underground but a few species deposit casts on the surface. This can spoil the appearance of fine lawns, particularly if the casts get squashed and spread over the surface by trampling feet or lawnmowers. This can create places where mosses and lawn weeds can establish.

Worm casts can be distinguished from soil brought up to the surface by other lawn animals, such as ants, mining bees and moles, by the fine muddy nature of the excreted soil. Freshly deposited worm casts often have a coiled appearance.

Control

Non-chemical control

Worm casts can be broken up and dispersed with a wire rake, using it with the teeth facing upwards and moving the rake from side to side over the lawn surface. This is only possible when they are dry. The deposition of worm casts tends to peak in autumn and early spring, and in mild winters can continue throughout the winter. There are often very few days during autumn-spring when worm casts are dry enough to be dispersed. It is best to keep off lawns as much as possible when worm casts are a problem during that period.

Chemical control

Earthworms are much less abundant in acidic soils, so in theory reducing the soil pH should help to reduce a worm cast problem. Products containing sulphur, such as Cast Clear® are offered for this purpose, these products do not kill worms but deter them from moving through treated areas. They often need repeated application to remain effective. Other products are also available to lower soil pH. If the soil is alkaline, i.e. above pH7, it would be difficult to make the soil sufficiently acidic to deter earthworms. If the soil is already acidic, i.e. below pH7, there is a possibility that repeated applications of sulphur might make the soil too acidic and adversely affect grass growth. It is easy to test soil to check the pH.

There are no pesticides that kill earthworms currently to available to home gardeners for controlling worms.

Biology

Earthworms are hermaphrodite animals that pair up to exchange semen. Eggs are deposited in the soil in lemon-shaped egg sacs. These eggs hatch into small worms similar in appearance to the adults they gradually increase in size as they develop. Different species of earthworm occupy different habitats. Some live in compost heaps or other accumulations of rotting organic matter, while others live in the soil. Some species live near the soil surface while others occur deeper underground. In most situations the activities of earthworms improve soil structure.